The present invention relates generally to a rimfire cartridge system, including a rimfire cartridge and to a method of making a rimfire cartridge, and more particularly to an improved rimfire cartridge having a primer free of toxic metals, for ammunition or industrial powerloads used in power-fastening tools to serve as a gas energy source for driving metal studs, fasteners and the like.
Rimfire cartridges heretofore have generally used priming compositions that produce a toxic gaseous exhaust product which includes compounds of lead, antimony or barium. Growing concerns about the effect on human health of these toxic exhaust product chemicals have led to investigations of new primer compositions. A desirable primer composition would have acceptable ignition properties and an impact sensitivity comparable to conventional primer compositions, while eliminating or reducing the undesirable chemical species in the exhaust product. Nontoxic exhaust product priming compositions are especially desirable for use in enclosed or inadequately ventilated places, such as indoor target ranges for ammunition, or enclosed construction sites for industrial powerloads.
The exhaust composition of a primer depends greatly upon the chemical system of the primer formulation. For example, nearly all of the current small arms primer formulations are based upon the impact-sensitive primary explosive, lead styphnate. The exhaust products of a lead styphnate primer formulation contain toxic lead or lead compounds. Small arms primer formulations also include an oxidizer component and a fuel component, with the conventional formulations having a barium nitrate oxidizer and an antimony sulfide fuel. Upon firing a conventionally primed rimfire cartridge, the barium nitrate and antimony sulfide also form undesirable gaseous toxins.
The formulation of a new lead-free, low toxicity exhaust primer mixture requires the elimination of the conventional substances used for the primary explosive, fuel and oxidizer. These components must be replaced with chemicals serving these same functions in the primer mixture to provide a new formulation. Such a new formulation must perform comparably with the former compositions, especially in the areas of impact sensitivity, thermal output and ignition characteristics.
A number of earlier investigations have focused on the primary explosive diazodinitrophenol, also known as "DDNP" or "dinol," (hereinafter "dinol") as a replacement for lead styphnate. While as an explosive dinol possesses certain desireable attributes, such as its nontoxic exhaust products of nitrogen, carbon oxides and water vapor, it also suffers various formulation difficulties. Additionally, while the impact sensitivity of dinol is roughly equivalent to that of lead styphnate, the sensitivity of dinol to friction is much less. Furthermore, dinol has a significantly higher detonation velocity than that of lead styphnate.
Other lead-free primer compositions have been proposed. One primer formulation using dinol is described in U.S. Pat. No. 4,363,679 to Hagel et al. The Hagel et al. formulation has a smokeless propellant, a titanium fuel, and a zinc peroxide oxidizer. Another primer formulation using dinol is disclosed in U.S. Pat. No. 4,608,102 to Krampen et al., which uses manganese dioxide as the oxidizer.
U.S. Pat. No. 4,674,409 to Lopata et al. (hereinafter, "Lopata") discloses a non-toxic, non-corrosive, lead-free rimfire ammunition cartridge. The primer mixture of Lopata consists essentially of manganese dioxide (MnO.sub.2), tetracene, dinol and glass. The Lopata priming mix may include 10-40% by weight manganese dioxide, 25-40% by weight dinol (dependent upon the amount of tetracene, such that the combined weight percentages of dinol and tetracene are within the range of 40-60%) and 10-30% rimfire glass. The mixture is made by a wet process, where timer is spun into the interior rim of the casing. A 13% nitrated nitrocellulose foil sheet of a compacted propellant is located adjacent the primer composition to hold it in place for reliable ignition upon detonation of the primer. A lead-free metallic bullet, preferably of copper, is mounted within the open end of the casing.
Lopata's requirement of a separate foil disk which is inserted or pressed into contact with the priming mixture is considered to be a disadvantage for several reasons. First, the completed Lopata cartridge requires one whole extra part, i.e., the foil disk, which must be ordered, inventoried, handled and separately assembled into the finished cartridge. This extra foil disk part not only adds material cost to the overall cartridge, but it also increases the overhead and labor costs associated with material ordering, storage and handling.
A more detailed explanation of the Lopata cartridge is believed to be disclosed in Technical Report ARCCD-TR-87003 prepared for the U.S. Army Armament Research, Development and Engineering Center, Close Combat Armament Center, Picatinny Arsenal, N.J. by Raymond Brands, entitled "Elimination of Airborne Lead Contamination from Caliber 0.22 Ammunition," published in June 1987. On page 4 of this report, it states, "A thin layer of nitrocellulose foil was added to bond the primer mixture in place and provide additional ignition energy." The test results listed in this report are rather poor, showing a large number of misfires, and a follow-up program was recommended to complete the project. These disappointing results probably arose from a number of factors, not the least of which would be the use of manganese dioxide, a low oxidizer ratio and the thin foil seal. The degree of success of the Lopta cartridge is perhaps best indicated by the fact that the assignee of this patent apparently has no product currently on the market covered by the Lopata patent.
A lead-free primer composition is disclosed in U.S. Pat. No. 4,963,201 to Bjerke et al. (hereinafter "Bjerke"), which is herein incorporated by reference for the teachings and disclosures therein. The co-inventors of the invention illustrated herein are among the co-inventors of the Bjerke patent and they are also employed by the assignee of both the Bjerke patent and the subject matter described herein. The Bjerke patent discloses a lead-free primer composition for use in the cup-like primers of centerfire ammunition. The Bjerke primer composition comprises dinol or potassium dinitrobenzofuroxane as the primary explosive, nitrate ester as ,the fuel, and strontium nitrate as the oxidizer.
These prior patents focused on combinations of primary explosives, fuels, and oxidizers which would perform comparably to the conventional small arms primer compositions without producing potentially harmful exhaust products. However, these new compositions had varying degrees of success, mainly because they differ radically in chemical ingredients from the conventional lead styphnate compositions. Consequently, the new compositions possessed to some degree different thermodynamic characteristics than the conventional primer compositions. Moreover, with the exception of the Lopata patent discussed above, these compositions were developed specifically for centerfire ammunition applications, rather than for rimfire applications.
Rimfire ignition differs significantly from centerfire ignition so it is apparent that a primer composition which is suitable for centerfire cartridges may not perform adequately in rimfire applications. A comparison of rimfire and centerfire cartridges and their manners of detonation will clarify this.
For a rimfire cartridge, the primer mixture is deposited in an integral annular rim cavity in the interior of the case head. For a centerfire cartridge, the case head has a pocket for receiving a replaceable centerfire primer. A replaceable centerfire primer has a separate metal cup into which the primer mixture is placed and dried. The centerfire primer cup may then be equipped with an anvil to aid in detonation. The completed primer is then seated in the pocket of the centerfire case head.
For both rimfire and centerfire cartridges, after the primer is in place a propellant, which is commonly known as gun powder, is added to the casing. For ammunition purposes, a bullet is then seated and crimped at the open mouth of the casing to complete the cartridge. For a rimfire industrial powerload, the open mouth of the casing is sealed closed by crimping the casing mouth shut.
In use, for centerfire ammunition, a firing pin strikes the replaceable metal cup containing the primer. For rimfire ammunition, a firing pin strikes the casing rim. Rimfire casings are not intended to be reusable, but centerfire casings which receive replaceable primer cups may be reused. In both rimfire and centerfire cartridges, the impact force of the firing pin detonates the primer. The detonated primer ignites to provide a resultant thermal output energy pulse of gas, thermal energy and hot particles which in turn ignites the propellant. The distribution of impact force from the detonated primer to the propellent is quite different in the rimfire and centerfire configurations.
During centerfire detonation, the primer ignition takes place within the primer cup. The resultant gas expansion and thermal pulse are directed toward the propellant charge through a flash hole in the pocket of the centerfire casing.
During rimfire detonation, the pinching action of the firing pin permanently deforms the casing rim at a point near the outer edge of the case head. The rimfire primer ignites at this pinching point of impact then combusts very rapidly around the interior of the annular rim. The resultant gas expansion and thermal pulse in the rimfire case head ignite the propellant charge.
Since a rimfire casing is not indexed within the firing chamber, the firing pin may strike the casing anywhere along the 360.degree. circumference of the casehead. If the primer is not evenly distributed around the interior circumference of the casehead, the cartridge may malfunction, creating an insufficient or an excessive energy pulse. An excessive energy pulse can cause premature detonation of the propellant, or cause the bullet to move prematurely or a powerload crimp to open prematurely. An insufficient energy pulse produces poor ignition and a subsequent low rate of burn for the propellant, which could cause a misfire or other undesirable "squib" conditions.
In earlier studies, we, the inventors of the invention illustrated herein, found that friction forces play a more important role in the impact sensitivity for rimfire applications than for centerfire applications. This factor is exemplified in the conventional lead styphnate formulations where it has been determined that a frictionator or physical sensitizer, such as ground glass, is necessary to achieve the requisite impact sensitivity for rimfire use. Thus, a primer formulation which meets the sensitivity requirements for a centerfire application very often exhibits extremely poor impact sensitivity for a rimfire application.
Thus, a need has existed for an improved lead-free primed rimfire cartridge system for ammunition and industrial powerloads, which overcomes and is not susceptible to, the above limitations and disadvantages.